[Qt-interest] signals and slots....revisited...

[Bo Thorsen]

Den 14-03-2011 09:22, sarvesh saran skrev:
> Hi Bo,
>
> thank you for your answer. There was one question that is still
> unanswered...
>
>     5. QT stores the names slots and signals of an object in its
>       meta object? in what format.....<name,function pointer> ?
>
>
> /Something like it, yes. But you don't need to know. Just use the SLOT
> and SIGNAL macros to access them./
> /
> /
> I would like to know...at least at a higher abstraction. The SLOT and
> SIGNAL macros would generate a string representation of the signal and
> the slot containing the name, arguments etc (kind like a mangled C++
> method name?). These names are then looked up (where?) and the
> appropriate slots are called ( i guess through the meta
> object?..qt_metacall).

I did this:

QLOG_DEBUG() << "Slot:" << SLOT(onFirmwareUpdateAvailable(QString)) << 
"signal:" << SIGNAL(customContextMenuRequested(QPoint));

And got this output:

Slot: 1onFirmwareUpdateAvailable(QString)
signal: 2customContextMenuRequested(QPoint)

These really are just strings with a 1 for a slot and 2 for a signal. 
connect() compares these to the lists in the generated moc code and 
finds the proper signal or slot.

> I want this info for a variety of reasons...i was under the belief that
> signals and slots involve function pointers until some one very recently
> contradicted this...so i would like some clarification as to how the
> signals/slots are mapped,looked up and called?

You need to read the generated moc code.

> On Mon, Mar 14, 2011 at 12:40 PM, Bo Thorsen <bo at fioniasoftware.dk
> <mailto:bo at fioniasoftware.dk>> wrote:
>
>     Hi Sarvesh,
>
>     Den 13-03-2011 19:00, sarvesh saran skrev:
>
>         I have been a Qt programmer for quite some time now and i understand
>         most of the general features of Qt. I am still confused  about how
>         signals and slots work...
>
>         this is what i know (correct me if i am wrong)
>
>            1. slots are like ordinary member functions.however they can be
>
>               called via signals irrespective of their access specifier.
>
>
>     Yes. A slot is just a method.
>
>            2. emit is a macro that expands to nothing.
>
>
>     Correct. Your code would work without "emit" in front of signals,
>     but it's useful for Qt programmers to communicate that this is a
>     signal emit, so you have no way of knowing how long time it will take.
>
>            3. when you write emit mysignal() you are just calling a function
>
>               called mysignal which has been implemented in the
>         generated moc file.
>
>
>     Correct.
>
>            4. for every object (instantiated from a class containing
>         Q_OBJECT
>
>               macro and inheriting QObject) an addition meta object is
>               instantiated ?
>
>
>     Only if your QObject subclass also have the Q_OBJECT macro.
>
>     class X : public QObject { ... }
>
>     If you don't have Q_OBJECT in the ... there is no additional meta
>     object.
>
>            5. QT stores the names slots and signals of an object in its
>           meta
>
>               object? in what format.....<name,function pointer> ?
>
>
>     Something like it, yes. But you don't need to know. Just use the
>     SLOT and SIGNAL macros to access them.
>
>            6. connections between objects are made at runtime...how??
>         not sure
>
>               how and where a slot is searched for....
>
>
>     The SLOT and SIGNAL macros generates a string based on name, type
>     (slot or signal) and arguments. Then it does a string compare until
>     it finds the one it's looking for.
>
>            7. but the arguments in the connect call are matched at
>         compile time?
>
>
>     No, it's all runtime. If you need compile time matching, boost
>     signals is the solution.
>
>            8. while connecting, only the arguments of the slot is
>
>               checked...surplus arguments are discarded.
>
>
>     Correct. You can have fewer arguments on the slots than there are on
>     the signals, and the generated moc code will just disregard those.
>     But you can't have more arguments on the slots than there are on the
>     signals.
>
>            9. SIGNAL and SLOT are macros......not sure what their
>         purpose is....
>
>
>     They generate the string from the slot/signal signature.
>
>
>         i tried to look into the generated moc file but things are still
>         murky..any help will be greatly appreciated.
>
>
>     I hope this helps :)
>
>     Bo Thorsen,
>     Fionia Software.
>
>     --
>
>     Expert Qt and C++ developer for hire
>     Contact me if you need expert Qt help
>     http://www.fioniasoftware.dk
>
>


Bo Thorsen,
Fionia Software.

-- 

Expert Qt and C++ developer for hire
Contact me if you need expert Qt help
http://www.fioniasoftware.dk